In conventionally known ranging apparatus and methods of this kind, pulsed measurement light (such as laser light) is emitted toward the object of measurement, the time it takes for the light to be reflected back from the object of measurement and received is measured, and the distance to the object of measurement is calculated from this elapsed time and the propagation speed of the laser light. However, when an object of measurement is thus irradiated with pulsed laser light and the light reflected back from the object of measurement is then received, what is received is not only the reflected laser light, but also natural light and so forth, which becomes noise light. This noise light is difficult to distinguish from the light reflected back from the object of measurement, so the problem is that it is difficult to measure distances accurately.
When ranging is performed in this way, as long as there is no change in the position of the object of measurement, the reflected light from the object of measurement is always received in a fixed length of time after the emission of the measurement light, but the timing at which noise light is received is random. In view of this, a method has been proposed whereby a frequency count is performed corresponding to distance (or to elapsed time) when the pulsed measurement light is emitted toward the object of measurement and the reflected light satisfies a specific condition for each emission, the frequencies counted for all of the measurement light emissions carried out repeatedly are added up to produce a frequency distribution table (histogram) corresponding to distance, and the distance at which the total count in this frequency distribution table is at its maximum is considered to be the distance to the object of measurement.
In the frequency distribution table produced as above, the timing at which the reflected light from the object of measurement is received is always constant, and the count is relatively high at the distance (or elapsed time) indicating this position. However, since the timing at which noise light is received is random, a frequency count corresponding to variously changing distances (or elapsed times) is performed for every frequency count carried out repeatedly, and the summed count is relatively low at the various distances (or elapsed times) in the frequency distribution table. Accordingly, if the distance corresponding to the point when a frequency increases in the frequency distribution table produced as described above (such as when it exceeds a specific threshold) is used as the distance to the object of measurement, the distance can be measured accurately by eliminating the effect of randomly occurring noise light.
Unfortunately, however, the following problems are encountered with this ranging method.
The first problem is as follows: Specifically, when the object of measurement is large and all of the laser light emitted from the ranging apparatus irradiates the object of measurement, it seems sufficient to calculate the distance corresponding to the point showing a high count in the frequency distribution table, but when the object of measurement is relatively small and the laser light also irradiates the area surrounding the object of measurement, so that reflected light from surrounding objects also comes back, or when there are a plurality of objects of measurement at varying distances within the laser light irradiation field of the ranging apparatus, so that reflected light comes back from each of the objects of measurement, there are a plurality of points showing high counts in the frequency distribution table. In cases such as these, a plurality of distances are calculated, but the issues of how to handle the plurality of distances and how to display them greatly affect the flexibility, functionality, and so forth of the ranging apparatus.
The second problem is as follows: Specifically, when the distance to the object of measurement is measured by irradiating the object of measurement with laser light through window glass, the laser light reflected by the window glass is also always received corresponding to the distance to this window glass. In general, the intensity of the light reflected from the window glass is low, but since the intensity of the reflected light received by a light receiver is greater for near objects than for objects farther away, the reflected light intensity detected by the light receiver is such that the reflected light from the object of measurement, which is farther away, is not readily discernable from the reflected light from the nearer window glass, so both of these may end up being counted, or just the reflected light from the window glass may be counted. In a case such as this, there is the danger that the count corresponding to the distance of the window glass will increase in the frequency distribution table, so that the distance corresponding to the position of the window glass will be mistakenly determined as the distance to the object of measurement. Similarly, if tree branches or the like are in front of the object, the reflected light from the tree branches is received, and these branches may end up being mistakenly judged to be the object of measurement.
The third problem is as follows: Namely, when a distance is measured by looking at the object of measurement through window glass, or when a distance is measured by looking at the object of measurement through tree branches, the reflected light from the window glass, tree branches, or the like, located in front of the object of measurement is also constantly received. Consequently, there is the danger that the frequency corresponding to the distance of these obstacles will be high in the frequency distribution table, so that the distance thereof will be determined as the distance to the object of measurement, the result being that the measured distance to the object of measurement is inaccurate.
Furthermore, the position at which the frequency in the frequency distribution table increases can be affected by the shaking of the user's hands when the ranging apparatus is held in the hands during measurement, atmospheric fluctuations in the measurement environment, and other such effects, which is a problem in that the measured distance is inconsistent, or frequencies with an extremely large peak appearing to be noise may occur in the frequency distribution table, and the direct use of these frequencies results in incorrect distance measurement. Also, when the distance to an object of measurement that spreads out longitudinally is measured, such as when the distance to a building is measured by looking obliquely at the walls of the building, it is difficult to determine the distance if the frequency increases over a wide range of distances.
The fourth problem is as follows: Specifically, the reflected light intensity from the object of measurement varies with the distance to the object of measurement, varies with the type of object of measurement (this is due to differences in the reflectivity of the object of measurement itself, for example), and also varies with the measurement conditions (such as whether the measurement is conducted in a bright or dark location, and whether the measurement is conducted under weather conditions that are clear, cloudy, rainy, foggy, etc.), so the counts in the frequency distribution table can fluctuate greatly depending on these factors. Consequently, it is extremely difficult to determine the level of frequency for the distance in the frequency distribution table to be taken as the position of the object of measurement.
In particular, to perform this determination by internal arithmetic processing in a ranging apparatus, the general practice is to preset a determination threshold, and automatically determine as the distance to the object of measurement the distance having a frequency that exceeds this determination threshold in the frequency distribution table. In this case, if the preset determination threshold is too high, there is a concern that no frequency over this threshold will be found, and the distance to the object of measurement cannot be specified, but on the other hand, if the determination threshold is too low, many frequencies over this threshold may be found, making it impossible to specify which of these is the distance to the object of measurement.
Furthermore, in a laser ranging apparatus such as this, the intensity of the reflected light weakens as the distance to the object of measurement increases, so high sensitivity is required to detect faint light, and an extremely short time must also be detected accurately. Avalanche photodiodes have been used as opto-electric conversion elements that meet these requirements.
Thus, avalanche photodiodes are often used when faint light needs to be detected at high sensitivity (high amplification) and at a high response rate.
However, because of their high sensitivity, avalanche photodiodes also have the drawback of low stability. Specifically, the proportion of current flowing with respect to light of a given intensity (referred to as the current multiplication factor) is a function of the reverse bias voltage being applied. Also, the current multiplication factor tends to increase sharply as the applied reverse bias voltage approaches the breakdown voltage. Thus, in order to raise the light detection sensitivity, it is preferable to use a voltage close to the breakdown voltage as the reverse bias voltage to be applied.
However, temperature affects the breakdown voltage, so if the reverse bias voltage is close to the breakdown voltage, a change in the breakdown voltage will greatly alter the current multiplication factor.
This situation is illustrated in FIG. 14, which is a graph of the relationship between the reverse bias voltage and the current multiplication factor (a value indicating the amount of current flowing when a given amount of light comes in). When this relationship is as indicated by the solid line, if the reverse bias voltage is set at V0, then the current multiplication factor is α0. However, when a change in the breakdown voltage causes this relationship to shift as indicated by the broken line, the current multiplication factor changes to α0′.
When this happens, if the avalanche photodiodes are used with a laser ranging apparatus to detect reflected light from an object of measurement, the output from the detector changes independently of the amount of reflected light, producing an error in the measurement timing for received reflected light, and in extreme cases measurement may not even be possible.
Accordingly, a device that would keep the temperature constant within the detector was added to conventional units, or the avalanche photodiodes were used at a lower current multiplication factor, so that the avalanche photodiodes would operate stably. In the former case, the cost of the apparatus increased by the cost of the device used to keep the temperature constant, and in the latter case, because there was a limit to the amount of light that could be detected, the measurable distance became shorter when the avalanche photodiodes were used with a laser ranging apparatus.